Differentiate between growth hormones and inhibitor hormones.

1. They bring growth.

2. They stimulate cell division.

3. They bring opening of stomata.

4. They check abscission of leaves except ethylene.

5. They break seed dormancy.

6. They induce formation of fruits and flowers in all types of plants.

7. Gibberellins overcome vernalization.

Inhibitor hormones :

1. They retard growth.

2. They inhibit cell division.

3. They bring closing of stomata.

4. They bring abscission of leaves.

5. They induce seed dormancy.

6. They generally do not induce fruit and flower formation.

7. They have no impact.

Positive role of ethylene are as follows:

1. Breaking of dormancy : It breaks dormancy of storage organs.

3. Root initiation : In low concentration ethylene helps in root initiation, growth of lateral roots and root hairs.

4. Fruit ripening : It aids in ripening of fruits. C Ethylene is used to induce artificial ripening of these fruits e.g. Citrus, Apple, Mango, Banana etc.

5. Flowering : It stimulates flowering in pineapple and related plants though in other cases the gaseous hormone causes fading of flowers.

Negative role of Ethylene :

1. Growth : It inhibits longitudinal growth and stimulates transverse growth.

2. Senescence : It hastens the senescence of leaves and flowers.

3. Abscission : Abscission of various parts is stimulated by ethylene which induces the formation of hydrolases.

4. Apical dominance : Ethylene promotes apical domiance and prolongs dormancy of lateral buds.

Plant growth has following three phases

(i) Meristematic Phase - This phase is characterised by constantly dividing cells. The cells are rich in protoplasm, possess large conspicuous nuclei. Their cell walls are primary in nature, thin and cellulosic with abundant plasmodesmatal connections. 

(ii) Elongation Phase - The cells of this phase elongate. Increased vacuolation, cell enlargement and new cell wall deposition are the characteristics of the cells in this phase.

(iii) Maturation Phase - The cells in this phase stop dividing and undergo differentiation and maturation.The cells of this zone, attain their maximal size in terms of wall thickening and protoplasmic modifications.

Types of plants depending upon photoperiodism :

1. Short day plants : These plants produce flowers in winter. They are also called long night plants e.g. Chrysanthemum, tabacco etc.

2. Long day plants : These plants produce flowers in summer, when the days are long and duration of light is above critical point e.g. Melon, Momordica, Water-melon, Lady's finger plant etc.

3. Short long day plants : These plants need short photoperiod for initiation and long photoperiod for blossoming of flowers e.g. Rye, Campanula etc.

4. Long short day plants : These plants need long photoperiod for initiation and short photoperiod for opening of floral buds. They give flowers between summer and winter e.g. Cestrum, Bryophyllum etc.

5. Intermediate plants : These plants need definite photoperiod for flowering e.g. wild kidney bean.

6. Day neutral plants : These plants produce flowers in both winter and summer seasons. They do not need any specific photoperiod e.g. Sunflower Tomato etc.

1. These are plants which require less exposure to light for a period less than the critical duration.

2. If night is interrupted by light then there is no flowering.

3. These need light of short wave lengths whose value is below critical point.

4. These produce flowering in winter.

7. If days are interrupted by darkness then no effect.

Long day plants :

1. These are plants which require the exposure to light for a period exceeding a well defined critical duration.

2. If night is interrupted by light then no such effect on flowering.

3. These need light whose value is above critical point.

5. These produce flowers in summer.

6. If days are interrupted by darkness then no flowers.